The overall aim of the proposed research is to further analyze serologically the chromosome region in man which includes and surrounds the major histocompatibility locus (HL-A). The program will be directed 1) to extending the definition of the HL-A locus and 2) to detecting the lymphocyte defined (LD) non-HL A loci by serologic means. A search will be isntituted for human antisera which identify 1) new alleles at the HL-A loci and 2) alleles governing the I region (MLR, Ir, GVHR) expressed on the surfaces of thymus derived lymphocytes (T cells) and/or bone marrow derived lymphocytes (B cells). The lymphocyte cytotoxicity test and an antigen binding assay will be used with peripheral blood cells, including separated cell subpopulations. The interaction of the various serologically defined antigen systems will be correlated with disease susceptibility, with immune response to various antigens and with the outcome of organ transplantation.